UWB refers to a radio communications technique fundamentally different from all the other radio frequency communications systems, referred to as “narrowband systems”. Without entering into excessive details, well known to those skilled in the art, modulated low-energy pulses of very short duration, typically less than one nanosecond are used to transmit data, whereby the occupied bandwidth has very broad values. According to a definition of the U.S. Federal Communications Commission (FCC), a UWB system is a radio system having a bandwidth greater than 20% of the center frequency measured at the −10 dB points, or, alternatively, having an RF bandwidth greater than 500 MHz. On Feb. 14, 2002, the FCC allocated limited use of spectrum between the interval 3.1-10.6 GHz for signals operating in UWB systems (in short, UWB signals).
The main concern regarding a UWB system is that it occupies a portion of spectrum wherein other narrowband systems already operate, so a regulation is necessary in order to avoid coexistence (e.g., interference) problems. Therefore, a regulatory authority like the FCC has to set strict limitations in the maximum emission for UWB signals, thus guaranteeing protection to the already existing and deployed radio services. UWB signal transmissions, following the FCC rules, must have a power spectral density below the involuntary electromagnetic emission level.
A UWB transmitter has to generate UWB pulse signals whose spectrum is compatible with the regulations, both in terms of the frequency interval and of the maximum emission.
U.S. Pat. No. 6,515,622 discloses an antenna system making use of UWB pulse signals, and describes a technique for generating them based on step recovery diodes. However, this technique is not compatible with integrated circuit architectures, and is not adapted to control the shape of the generated pulse signals.
A typical category of UWB pulse signals that are compatible with the regulatory prescriptions of the national/supernational authorities and that can be easily varied in shape consists of the family of the monocycle wavelets. A Gaussian monocycle wavelet is a short-duration wave having, in the time domain, a shape represented by a Gaussian derivative.
A first solution known in the art for generating a monocycle wavelet is described by H. Kim, D. Park and Joo Y., in “Design of CMOS Shotlz's Monocycle pulse generator”, IEEE 7803-8187-4 2003, p. 81. According to this solution, a monocycle wavelet of the second order (i.e., corresponding to the second time derivative of a Gaussian pulse) is generated by differentiating a signal having the shape of a hyperbolic tangent by means of a squarer circuit coupled to a high-pass filter. However, this solution is adapted to low-frequency applications only. Moreover, since the generated monocycle wavelet is of the second order, its spectrum does not properly match with the spectral interval allowed by the FCC, unless a further filtering operation is performed. However, the further filtering operation increases the pulse duration, resulting in a degradation of the transmission.
An alternative solution for generating a monocycle wavelet is described by J. F. M. Gerrits and Farserotu, in “Wavelet generation circuit for UWB impulse radio applications”, Electronics Letters, 5 Dec. 2002, Vol. 38 n. 25, p. 1737. The document describes a circuit capable of approximating a monocycle wavelet of the second order by means of sums and differences of signals with the shape of a hyperbolic tangent. This solution has substantially the same drawbacks as the previous solution.
Another technique for obtaining a monocycle wavelet, or at least to obtain an approximate version thereof, consists of modulating a sinusoidal signal (the “carrier signal”) with a modulating signal pulse of suitable shape (in the time domain), thereby obtaining a modulated sinusoidal carrier monocycle with envelope corresponding to the shape of the modulating signal pulse. An advantageous method for obtaining a monocycle wavelet having a spectra suitable for a UWB transmission under e.g. the FCC rules, consists of using modulating signal pulses having the shape that is as close as possible to a Gaussian.
I. Gresham and A. Jenkins describe in “A Fast Switching, High Isolation Absorptive SPST SiGe Switch for 24 GHz Automotive Applications”, 33rd European Microwave Conference, Munich 2003, pag. 903-906, a UWB pulse generator circuit that generates a square-envelope modulated monocycle by multiplying a high frequency sinusoidal wave by a square modulating pulse. This circuit includes a switch circuit having a very short switching time, being based on the E2CL architecture. Although this circuit is adapted to operate at high frequencies, the spectrum of a square-envelope modulated monocycle can not be entirely confined in the spectral interval allowed by the FCC, because it is characterized by having (ideally infinite) lateral lobes.
The International Application WO 0139451 describes a UWB data transmission system that generates low level voltage pulses. The shape of the low level voltage pulses can be varied by means of an adjustable shaping filter. Once shaped, the voltage pulses are used for modulating a sinusoidal signal, in such a way to obtain the UWB pulse signals necessary for the transmission of data. This solution allows modification to some extent the shape of the low level voltage pulses, and consequently modifying the shape of the UWB pulse signals spectrum. However, the shaping capability given by the shaping filter is limited, and the spectrum is scarcely controllable, which is also worsened by the circuit complexity of the shaping filter itself.